Superconducting magnets presently in use in MRI systems are basically cylindrically shaped with a magnet in a cryostat and having a bore tube external to the cryostat through which the generated magnetic flux runs.
A patient in a prone position is placed in the bore tube virtually coaxial with the longitudinal axis of the cylindrical magnet. Thus, the magnetic lines of force run parallel to the longitudinal axis of the patient.
Among the drawbacks of the present cylindrical magnets is that the patient is wholly within the cylinder; i.e., from the time the patient is positioned in the bore tube, the doctor does not have ready access to the patient. The lack of access presents a severe problem when the patient is in critical condition and needs continual or emergency aid from the doctor. Present systems require the patient to be removed from the bore tube of the magnet to enable access by the doctor. Such removal of the patient from the bore tube could be critical and in fact fatal in certain emergency cases.
The cylindrical type magnets have an additional shortcoming. The magnetic lines of flux travel largely through air which offers a much higher reluctance path than does a magnetizable material such is iron or steel. Consequently, the magnetic field is inherently weakened by the large proportion of the travel of the flux that is passing through air.
In the past, attempts have been made to provide more iron in the path of the magnetic flux. This has been attempted by using C-frame magnetic sections or what are known as "H-frame" magnet sections. In the C-frame construction, an iron or steel yoke passing through an electro magnet carries magnetic flux to oppositely disposed pole pieces. Flux crossing the air gap between these pole pieces is guided by pole shaping sections of electrical coils to form homogeneous zones suitable for MRI studies.
In general the C-frame type magnets have some fundamental problems which have inhibited their use for MRI studies. Among the problems are:
1) The C-frame magnet is mechanically imbalanced by magnetic forces applied between the poles. With the high powered magnets used there is actually some small but non-negligible movement when the superconducting coil of the magnet is conducting. The magnetic attraction upsets the homogeneity of the flux between the pole pieces; and PA1 2) Stray field performance in the C-type magnet is much worse than the stray field performance using state of the art devices presently available for controlling the stray fields generated by cylindrical high field magnets. PA1 a pair of spaced apart oppositely disposed walls of magnetizable material, PA1 a first pole piece extending from one of said walls toward second pole piece extending from the other of said pair of oppositely disposed walls, PA1 an air gap between said first and said second pole pieces, PA1 a superconducting coil on at least one of said first or said second pole pieces for use in generating a magnetic field between said pole pieces, PA1 said air gap being sufficiently large for receiving a subject therein to be imaged using magnetic resonance, PA1 gradient coil means positioned to cause three orthogonal gradients in said air gap, PA1 RF coil means for transmitting RF magnetic fields into said air gap and for receiving free induction decay (FID) signals from the subject in the air gap, and PA1 magnetic circuit closing means connecting said oppositely disposed walls at opposite ends thereof but enabling ready access to said subject in said air gap between said pole pieces.
Another theoretical shape design of a magnet which in theory can improve over the presently available cylindrical magnets is the H-frame magnet. Such magnets provide a double path of magnetic material for the magnetic lines of force. The H-frame has, however, in practice a number of serious problems when used in high field superconducting devices. With the H-frame, a larger magnet is needed to accommodate the cryostat, the flux path is improved, but at the expense of the larger magnet and the doctor still does not have ready access to the patient. In addition, the H-frame magnet is imbalanced magnetically and thus, there is great difficulty in obtaining homogeneous fields with the H-frame magnet.
If the magnet is to be balanced by adding a second cryostat, for example, and another set of superconducting coils, then the magnet would have to be even larger and more expensive than the single coil H-frame magnet suggested todate.
Accordingly, those skilled in the art are still searching for new and approved magnets for use in MRI systems.